1. Field of the Invention
This invention relates to the manufacture of curved glass panes of laminated glass. The panes contain two glass sheets and an inner, intermediate layer, typically a thermoplastic film. At least one of the two glass sheets is provided with a printed surface which is to be baked-in prior to the bending of the two glass sheets, as a pair, into the final shape.
2. Discussion of the Background
Laminated glass window panes, particularly for automobile windshields or rear windows, often have printed surfaces. The printed surfaces may be, for instance, colored films, decorative frames, and heating resistors. When the printed material is to be baked-in (i.e., when using a printing ink, or an enamel or enamel-like substance), the printed material is referred to as a "baking finish". It is desirable to place such a baking finish on an internal surface of the glass pane, that is, on a surface bearing against the thermoplastic film.
However, the application of a baking finish and the subsequent baking-in process presents problems when a curved glass laminated pane is manufactured. It has previously not been possible to carry out the baking-in process during the course of the bending operation. This is because baking finishes contain organic constituents which form gaseous reaction products at the baking temperature. The gases cannot escape from the space between the two glass sheets. Furthermore, there is an increased risk that in the melting of the baking finish the two glass sheets will stick together.
The bending of glass sheets in pairs with simultaneous baking-in of an enamel film is described in EP 0 013 970 B1. This method is only for the case in which the baking finish is applied in a central partial region of the glass pane. During the baking and bending of a pair of glass sheets, each sheet opens out somewhat because of the higher temperature of the outer surface compared with the temperature of the inner surface. This allows the reaction gases to escape and prevents the two glass sheets from sticking together. If, in contrast, the baking finish extends over fairly large regions, this method can no longer be used.
In the case of fairly large printed areas on one of the inner faces of a laminated glass pane, the normal procedure is as follows. A single glass sheet is provided with a printed baking finish. This sheet is then subjected, alone and in the plane state with the printed surface upwards, to a heat treatment process. The baking finish, for instance, an enamel film, is melted and baked in. After cooling, this glass sheet with the baked-in enamel film is brought together with the second glass sheet and the pair of glass sheets are together bent into their final shape. In this joint bending process, reaction products from the baking finish are not given off. Sticking together of the sheets is avoided by powdered parting agents applied to the internal surfaces. These parting agents are washed off after the bending and before further processing to form the laminated pane. This method is known, for example, from DE 39 20 573 A1, which describes baking-in an enamel film on a flat glass sheet in a preliminary process step.
However, such a process makes heavy demands upon baking furnaces suitable for this purpose, because the glass sheet with the printed surface not only must remain absolutely flat and without deformations, but also must not possess any internal stresses. Any deformation or internal stress in the baking process will lead to perturbing deformations in the later bending process. It has, for example, been found that even the transporting of the hot glass sheets on conveying rollers can lead to slight deformations with the result that, for example, windscreens produced by this technique cannot satisfy strict criteria with regard to their optical quality.